Effective in vivo delivery of active agents such as small molecule drugs, proteins, peptides, and nucleic acids represents a continuing medical challenge. Some active agents are recognized by the immune system, resulting in decreased efficacy. To address this issue, certain active agent formulations have incorporated polymers such as polyethylene glycol which was thought to cloak or mask the agent, thereby reducing its antigenicity and immunogenicity. However, even these “stealth” formulations have their limitations, including an inability to be repeatedly and frequently dosed, for example, over a period of days without loss of activity.
In addition, some agents or formulations when administered in vivo may interact with one or more cells or factors, potentially interfering with their functions, and ultimately resulting in adverse effects. Such adverse effects may limit the administration frequency and/or administered dose of the agent, or may preclude in vivo use altogether.